In the solar energy vacuum tube collectors, a vacuum seal must be present between the glass envelope tube and the metal absorber tube. The area between the glass envelope tube and the metal absorber tube must be evacuated to ensure thermal isolation. The purpose of the glass-to-metal joint is to provide a hermetic seal between the glass envelope tube and the metal absorber tube to maintain the necessary vacuum.
Because solar collectors operating in the field experience daily temperature fluctuations with temperatures reaching 400° C. The temperature fluctuations compounded by the mismatch in the thermal expansion coefficients of typical glass envelope tubes and the metal at the glass-to-metal seal result in cyclical thermal stresses in the glass envelope, particularly at the glass-to-metal joint and pose reliability problems.
In some glass-to-metal joints currently used in high temperature solar collectors, a housekeeper seal design is used. In the housekeeper seal, a very thin metal tongue with a comparatively high thermal expansion coefficient of about 5×10−6 K−1 in the temperature range of 25° C. to 350° C. is fused with a temperature-change-resistant glass with a considerably lower thermal expansion coefficient of about 3×10−6 K−1. The thermal stresses under thermal loads are then absorbed by plastic deformation of the metal tongue. The cyclical thermal stresses, however, frequently lead to undesirably high mechanical breakdown rates of the glass-metal joint of more than 4% per annum. This is a barrier to widespread introduction of solar technology in the power-engineering field.
In some conventional systems, one or more intermediary glasses are used to form the glass-to-metal joints. This solution is costly because it increases the engineering work, the disposal rate and manufacturing costs.
In other conventional solar energy vacuum tube collectors, also known as heat collector element (HCE), an absorber tube, generally a metal pipe, is provided within a glass envelope cylinder/tube. The absorber tube extends the entire length of the glass envelope and the ends of the absorber tube and the glass envelope are hermetically sealed together by a sealing assembly and the space between the glass envelope and the absorber tube is maintained in a vacuum. The sealing assembly generally comprises a tubular metal sleeve that is joined to the glass envelope on one end and joined to a flexible bellows structure on the other end. The flexible bellows structure is in turn joined to the absorber tube extending out from the glass envelope. One of the structural reliability concerns with this structure arises from the mismatch in the thermal coefficient of expansion (TCE) between the glass envelope and the metal sleeve at the glass-to-metal joint formed between the glass envelope and the metal sleeve. The mismatch in the TCE between the two materials forming the glass-to-metal joint generates cyclical stresses during the operation of the solar collectors and may result in failure of the glass-to-metal joint.
There has been attempts to match the TCEs of the glass envelope and the metal sleeve to improve the reliability of the glass-to-metal joint. However, there is a need for more improvement.
Thus, there is a need for improved material set that can form a more robust glass-to-metal seal.